In a sample holder of this kind one or more materials are simultaneously subjected, for the purpose of decomposition or analysis, to chemical treatment in which heat may be supplied to promote the reaction. An example of a sample material to be investigated is sewage sludge that is to be decomposed with highly aggressive acids. To accelerate the decomposition, i.e. the chemical process, it is usual to heat the sample material with the addition of acids or other decomposition agents by heating the sample holder with the sample material and the decomposition agent in a heating appliance, preferably by subjecting it to microwave radiation in a microwave oven.
The heating can result in a large excess pressure in the sample holder. To prevent damage to or explosion of the sample holder a valve is provided in an outlet opening that automatically opens if a predetermined internal pressure is exceeded.
In German patent application 38 39 901 a sample holder of the above-mentioned kind is described that comprises a bottom container part with a pot-shaped cylindrical lid that can be screwed in an inverted position on to the pot-shaped cylindrical bottom container part. In this arrangement the valve forms an integral part of a valve body that can be screwed on to a connecting piece that is located centrally on the lid and incorporates the outlet opening. The valve is situated in an outlet passage that passes through the valve body and includes a conical valve seat against which a valve member having a corresponding conical head can be prestressed from above by an amount such that when the internal pressure in the sample holder reaches a predetermined value the valve member lifts off under this internal pressure and frees the outlet opening. The valve member can move vertically in a plug that can be screwed from above into an open-topped recess in the valve body. The prestressing of the valve member is done through a thrust piece of elastic material, preferably silicone, that is located between the valve member and the plug. The plug can be screwed into the valve body in the manner of a press screw, by means of a turning member at its upper end, until the elastic thrust piece is compressed, thereby producing the prestressing force. The depth to which the plug is screwed in, and thus the prestressing force, can be determined by means of indicating marks on the valve body and/or on the turning member.
This known arrangement has proved to be practicable and useful. However, it is complicated and expensive to manufacture, since a special valve housing has to be made and fitted. The valve housing seated on the lid, or the valve body, also increases the height of the sample holder. Screwing the additional valve housing or valve body on and off is also regarded as a time-consuming operation. Apart from this, screw threads on the top and bottom parts of the container are expensive and make manipulation slow and difficult. Furthermore thick walls are necessary, leading to long cooling times and increased material requirements and weight. There is also a risk of damage to the sample holder by distortion and ageing, particularly in the region of the screw threads and with long decomposition times.
The heating of the sample material in the sample holder is preferably done in a microwave oven, in which several sample holders can be heated at the same time. To ensure uniformity of heating a rotary stand is provided in the oven on which the sample holders can be rotated about a vertical axis.
To provide resistance to the corrosive acids that can be generated in the course of the decomposition and to make microwave heating possible a chemically inert material that is transparent to microwaves is required. For this purpose a heat-resistant and chemically inert plastic material is very suitable.
In an arrangement known from DE-B-36 20 381, having a pot-shaped receiving part with a lid in the form of a disc resting-on its rim, the valve member is formed by the lid itself and a flat bearing disc of aluminum is arranged on the lid. To use a sample holder with a valve constructed in this way for the decomposition of sample material requires not only a stand in the oven but also a retaining means extending over the sample holder to hold the lid in its closed position. For this purpose a base plate is provided beneath the sample holder and a retaining plate that extends over it, the retaining plate having a press screw arranged to be screwed into it from above and coaxially with the sample holder so that its lower end presses vertically against the bearing disc and thus holds the valve in its closed position. Under excess internal pressure in the sample holder the lid can distort slightly and rock about the press screw. Oscillatory bending upwards of the lid will clearly occur in some position at its periphery. DE-B-36 20 381 does not say whether or not the bearing disc participates in this movement of the lid. It must be assumed that the aluminum bearing disc will likewise bend upwards and outwards under the increased internal pressure at some position on the periphery, so that the lid follows this movement and the excess pressure can blow off through the resulting gap. The internal pressure will obviously always seek out the weakest peripheral position on the lid and/or the bearing disc. At any rate, what will happen is a very indefinite opening, since both in the case of one lid in this peripheral region and also when there is more than one lid (simultaneous heating of several sample holders) blowing off will occur at different internal pressures as a result of differences in the bending strength of the lid or lids. This is undesirable, since at uncontrolled different internal pressures different decomposition reactions can occur and thus falsify the results of the decomposition or the analysis. With this known arrangement there is also the danger that the aluminum bearing disc may be permanently deformed either on screwing up the press screw or by the internal pressure, thus impairing the functioning of the sample holder through premature opening or making it unusable. A further disadvantage is that this known arrangement is not suitable for heating in a microwave oven.